Slaty-backed Gull L.schistisagus
13th January 2004 Choshi Chiba Japan
Note: Primaries of most juv/1w Slaty-backed Gulls are within the range from pale buff-brown to blackish brown, not jet black. But it is sometimes hard to read in some photographs because of light conditions.
The scapulars are mostly juvenile plumage. The primaries are not so blackish in the field.
An advanced example. Note the replaced new scapulars, old juvenile wing coverts, and brown primaries.
Note that the primaries are not real black, and have pale fringes.
The tail, in this individual, looks almost all-dark. Note that the secondaries look dark in contrast to the wing coverts, unlike Glaucous-winged Gulls.
2 fresher individuals.
A pale, advanced individual. This resembles somewhat Glaucous-winged Gulls. But its outer primaries and tail are dark in contrast to the rest. This type may suggest Glaucous-wingedXSlaty-backed as well, but I think this type is too common in Japan to be that, in comparison with the frequency of adults presumed to be the parentage.
A strangely contrasty individual for a Slaty-backed Gull. But, in the field, the primaries are not really blackish, and had pale fringes to the tips. The light is coming in from the left side in this situation, and misleadingly makes the primary and tertials dark.